In the art of preparing metal surfaces to receive paint, it is widely known that the formation of various conversion coatings such as coatings of phosphates, mixed phosphate-oxides, chromates, oxalates and arsenates, on the surface of the metal substantially increases the corrosion resistance and humidity resistance of the final painted surface. Likewise, the use of manganese phosphate coatings has been found to increase the lubricity of the metal surface. It is also well known that the effectiveness of these coatings is substantially increased by the application to the coated surface of a dilute solution of chromium and its various forms, i.e., chromic acid, hexavalent chromium and/or trivalent chromium; said solution optionally containing other ingredients such as phosphoric acid, a reducing agent for chromium such as formaldehyde or straight polyalcohols, wetting agents, pH adjusters and the like. Thus, in the commericial, high speed, production line processing of metal parts for painting, the predominant processes apply the phosphate or other conversion coating from an aqueous bath and, after water rinsing the formed chemical coating, it is subjected to the aqueous chromium rinse either by spraying or dipping. In many processes the chromium rinse step is then followed by an additional deionized water rinse to remove uneven concentrations of the chromium on the surface of the metal. While all of these processes increase the corrosion resistance of the metal surfaces, they all have a very serious drawback, namely, the fact that the chromium contained in both the chromium and water rinses cannot effectively be removed from these solutions. Consequently, even after extensive efforts have been made to remove the chromium, some chromium will be carried over in the effluent of a metal processing plant into the environment, thereby creating a serious problem.
It is, therefore, an object of the present invention to provide a composition and process for rinsing conversion coated metal surfaces to substantially increase the corrosion and humidity resistance of such surfaces, which composition and process does not pose a threat to the surrounding environment.
Another object of the present invention is to provide a composition and process for rinsing conversion coated metal surfaces to improve the surface's receptivity to subsequent adherent coats of paint, which composition and process does not pose a threat to the surrounding environment.
It is a further object of the present invention to provide a final rinse for conversion coated metals which rinse, while yielding coatings of superior corrosion resistance, can be easily and economically treated to yield relatively pure effluent from said process. Other objects will be apparent from the following description.